Occupy protesters have accused New York police officers of beating a woman and then neglecting her when she suffered a seizure after being handcuffed.

Cecily McMillan was arrested on Saturday night as police removed scores of demonstrators from Zuccotti Park, formerly the base of the Occupy movement.

The case, and the weekend's violent clashes, have fueled allegations that the NYPD is adopting brutal and intimidatory tactics to prevent the Occupy movement taking ground in the city in the way it did last September.

Bystanders are heard screaming at police to call an ambulance and remove her handcuffs, while a number of officers are seen standing around her convulsing body. Numerous witnesses told the Guardian that McMillan's head was unsupported throughout the incident and claimed her skull repeatedly struck the pavement.

The New York police department, meanwhile, has pointed to a separate video allegedly detailing the sequence of events leading to McMillan's arrest. Grainy footage shows an individual who the NYPD claims is McMillan swinging an elbow backwards and striking a police officer in the head.

McMillan, who faces charges of assault and obstructing governmental administration, was released on Monday afternoon after a judge denied a request from the district attorney that bail be set at $20,000.

During the court appearance, there was noticeable swelling and discoloration on McMillan's left eyelid. At times her breathing appeared labored. As she waited to approach the judge, a police officer at McMillan's side asked her supporters to have painkillers ready upon her release.

The district attorney accused her of "intentionally walking up to a police officer and elbowing him in the face", reportedly resulting in swelling and bruising to the officer's left eye. When the judge asked if the officer sustained further injuries, the district attorney replied: "Not that I'm aware of."

Witnesses to Saturday's police crackdown at Zuccotti Park said McMillan suffered heavy-handed treatment as she was taken into custody.

A woman who chose to identify herself as "Anne", said she was no more than 25ft away from McMillan when she was taken down by police.

"She was walking away from where the arrests were happening," she claimed. Anne said she did not witness the incident that precipitated McMillan's arrest, but said McMillan was quickly thrown to the ground.

Anne claims that "without a doubt, there was kicking and clubs being used" as police moved in on McMillan for what Anne estimated was at least 30 seconds to a minute.

"I'm 31 years old and I've never seen a violent act like that before in my life," Anne said. "It was horrifying to see."

"I became so upset that I had to walk away because I was crying," she added.

Jennifer Waller saw McMillan convulsing in the street after police took her into custody and shared a crowded jail cell with her on Saturday night. According to Waller, it was clear that McMillan was in "serious pain" before she suffered her apparent seizure. The two were among a few dozen protesters held at the park's east side.

"We were yelling: 'This woman needs medical attention'," Waller said. "Next thing I knew she was in the middle of the street writhing, having a seizure and that went on for more than 10 minutes."

Witnesses have claimed it took at least 17 minutes for an ambulance to arrive before McMillan was transferred to a local hospital. It remains unclear if the ambulance was summoned by police or bystanders. Upon her release, McMillan was transferred to the NYPD's midtown south precinct, where Waller says the conditions were cramped.

Using their arms to measure, Waller and her fellow detainees determined, "There were 21 women in a 6ft x 11ft cell."

Waller said there was "nowhere to lie down or sit".

"We just tried to make [McMillan] as comfortable as possible," Waller claimed. "She could not be touched. Her entire body was in pain."

"We just took turns holding her hand," she added. "She was just crying hysterically and showed us all her bruises all over her body."

"They told her at the hospital that she would be brought back to the precinct for 20 minutes and then she would be seeing her personal doctor," Waller claimed. "Twenty minutes turned into many hours."

The police confirmed that McMillan was eventually transferred to Bellevue hospital. The extent of her injuries and treatment remain unclear. Following her arrest on Saturday evening, McMillan's attorney was unable to speak to her one-on-one until Monday afternoon at approximately 2.15pm, shortly before she appeared before the judge.

On Monday, the Mass Defense Coordination Committee of New York City chapter of the National Lawyers Guild issued a statement regarding Saturday's police activity.

"The NYPD's egregious behavior toward protesters this weekend is merely one highly visible instance of their larger program of the arbitrary and brutal behavior that disrupts the health and welfare of communities city-wide," the statement read.

The MDCC called for an independent investigation into the department's practices. "We demand attention to these repressive, hostile and violent practices, including a federal investigation into the policies and practices of the NYPD under the auspices of Commissioner Kelly and Mayor Bloomberg. Without critical scrutiny of NYPD policy and practice, the safety of New York City's residents is more compromised than protected by the police."